In certain installations where emergency valve operation is required, it may be of critical importance that the valves be reliable and rapid in operation. For example, in nuclear power generating stations the rupture of a line containing feed water or coolant precipitates an emergency situation requiring isolation of the ruptured section by immediate closure of a valve. On the other hand, an emergency involving a reactor may produce a need for rapid opening of a valve as to douse the reactor or to flood a compartment. Such line ruptures could be precipitated by seismic forces, such as earthquake, or by explosion through sabotage or by missile. While such rapid valve operation may be required, it is also important that the valve closure member not impact with its seat with such momentum as to cause valve damage which would impair or prevent future valve operation. It is also vital concern that the valve actuator be reliable and capable in operation dispite damage or defects. It is further essential that the valve actuator be so constructed as to enable repair and replacement of parts quickly in order to minimize time of human exposure to the nuclear environment.